Optical fibre based distributed acoustic sensors (DAS) are known in the art. One high performance example is the iDAS™, available from Silixa Limited, of Elstree, UK. Further details of the operation of a suitable DAS are given in WO2010/0136809 and WO2010/136810, which also disclose that distributed acoustic sensors may be used for in-well applications, in that the acoustic noise profile can be used to measure the flow by noise logging at every location along the well. In addition, the noise spectrum can be used to identify the phase of the fluid.
However, one problem that arises in the use of DAS for flow monitoring is in fluid carrying structures where the flow is quiet, for example where the flow is substantially laminar, or with few eddies or other turbulent regions that cause noise. In such a case acoustic monitoring of the fluid carrying structure is unable to determine the fluid flow, or the fluid phase, due to the lack of input information into the sensor. Moreover, in fluid carrying structures where the flow is sometimes noisy and sometimes quiet, the monitoring of such structures with a DAS can result in large amounts of data, much of which is of little use when no noise is present.
Examples of flow carrying structures that are sometimes too quiet for conventional DAS monitoring are oil wells with low flow rates, and shale oil or shale gas wells. Even horizontal sections of piping can have quiet flow.